1. Field of the Invention
The present invention relates to a method of cutting a rare-earth alloy and more particularly, the present invention relates to a method of cutting a rare-earth alloy with a wire saw, which is obtained by fixing abrasive grains on a core wire.
2. Description of the Related Art
A rare-earth alloy is used as a material to make a strong magnet. A rare-earth magnet, obtained by magnetizing a rare-earth alloy, can be used effectively as a magnet for a voice coil motor, which is used to position a magnetic head in a magnetic recorder, for example.
In the prior art, a rare-earth alloy material (e.g., in the form of an ingot or a sintered body) is often cut by a technique of slicing the material with a rotating slicing blade, for example. In this slicing blade cutting process, however, the cutting edge is relatively thick, thus requiring a lot of machining allowance. As a result, the yield of the rare-earth alloy material is so low that it increases the cost of resultant rare-earth alloy products (e.g., rare-earth magnets).
A wire saw cutting process is known as a cutting method that requires smaller machining allowance than the slicing blade cutting process does. For example, Japanese Laid-Open Publication No. 11-198020 discloses that a hard and brittle material such as silicon, glass, neodymium or ferrite may be cut with a wire saw, which is obtained by fixing superabrasive grains on the outer surface of a high-hardness core wire with a bonding layer (which will be referred to herein as a “fixed abrasive wire saw”).
If a number of plates with a predetermined thickness can be obtained at the same time by cutting a rare-earth alloy material with such a fixed abrasive wire saw with small machining allowance, then the manufacturing cost of rare-earth magnets can be reduced significantly. However, nobody has ever reported that a rare-earth alloy could be cut successfully with such a fixed abrasive wire saw at a mass-producible level.
The present inventors carried out extensive research on this phenomenon and discovered the major cause of this problem in a significant difference in mechanical property between a rare-earth alloy produced by a sintering process (which will be referred to herein as a “rare-earth sintered alloy”) and silicon, for example. More specifically, a rare-earth sintered alloy includes an overall hard and brittle main phase (i.e., R2Fe14B crystal grains) and a grain boundary phase that causes ductile fracture. Accordingly, unlike a hard and brittle material such as silicon, the rare-earth sintered alloy is not so easy to cut. That is to say, compared with cutting silicon or any other hard and brittle material, higher cutting resistance is produced and a huge quantity of heat is generated, too. Also, the specific gravity of a rare-earth alloy is approximately 7.5, which is much higher than that of silicon or any other hard and brittle material. For that reason, cutting debris (or sludge), produced by the machining process, cannot be easily flushed away from the machined portion.
Thus, to cut a rare-earth alloy with high machining accuracy and efficiency, it is necessary to not only decrease the cutting resistance sufficiently but also efficiently dissipate the heat to be generated during the cutting process (i.e., efficiently cool the machined portion). Furthermore, it is also necessary to efficiently flush away the cutting debris produced by the cutting process.
For that purpose, by supplying the rare-earth alloy machined portion with plenty of highly lubricating coolant (which will also be referred to herein as a “cutting fluid”), the cutting resistance can be decreased and the heat generated during the cutting process can be dissipated efficiently. The present inventors discovered and confirmed via experiments that if a wire saw is wet with a sufficient amount of an oil coolant, then the traveling wire saw can supply a narrow machined portion with plenty of that coolant.
When such an oil coolant is used, however, it costs a lot to process its waste so as not to create any environmental damage and it is difficult to recycle the waste or cutting debris because the cutting debris is hard to separate from the waste. In view of these considerations, water (or an aqueous coolant) is preferably used as the coolant. However, if water is used as a coolant, then it is impossible to keep a sufficient amount of water deposited on the traveling wire saw because water has low viscosity (with a kinematic viscosity of 1.0 mm2/s). As a result, even if the wire saw was wet with water, a sufficient amount of water could not be supplied to the machined portion.
Japanese Laid-Open Publication No. 11-198020 discloses that by moving the wire saw through a coolant overflowing from a coolant vessel, the coolant can be kept deposited on the wire saw just as intended even in a situation where a fixed abrasive wire saw needs to travel at a high velocity (e.g., at 2,000 m/min). However, the present inventors discovered via experiments that even when a rare-earth alloy was cut with a wire saw traveling through such overflowing water (as disclosed in Japanese Laid-Open Publication No. 11-198020, for example), the abrasive grains still dropped off, the resin layer peeled off, or the wire saw snapped in a worst case scenario. These inconveniences also happened even when the wire saw traveled at a velocity of about 800 m/min.